The island of Utøya, where at least 80 people have died after a gunman opened fire at a summer camp. Photograph: Lasse Tur/AP

A Norwegian dressed as a police officer killed at least 80 people at an island retreat, police said early on Saturday. It took investigators several hours to begin to realise the full scope of the massacre, which followed an explosion in Oslo that killed seven and that police say was set off by the same suspect.

Police initially said about 10 people were killed at the camp on the island of Utøya, but some survivors said they thought the toll was much higher. Police director Øystein Mæland told reporters early on Saturday they had discovered many more victims.

"It's taken time to search the area. What we know now is that we can say that there are at least 80 killed at Utøya," Mæland said. "It goes without saying that this gives dimensions to this incident that are exceptional."

Mæland said the death toll could rise even more. He said others were severely injured, but police did not know how many were hurt.

A suspect in the shootings and the Oslo explosion was arrested. Though police did not release his name, Norwegian national broadcaster NRK identified him as 32-year-old Anders Behring Breivik and said police searched his Oslo apartment overnight.

A police official said the suspect appears to have acted alone in both attacks, and that "it seems that this is not linked to any international terrorist organisations". The official spoke on condition of anonymity because that information had not been officially released by Norway's police.

The official said the attack "is probably more Norway's Oklahoma City than it is Norway's World Trade Center."

The motive was unknown, but both attacks were in areas connected to the ruling Labour party government. The youth camp, about 20 miles northwest of Oslo, is organised by the party's youth wing, and the prime minister had been scheduled to speak there on Saturday.

The blast in Oslo left a square covered in twisted metal, shattered glass and documents expelled from surrounding buildings. Most of the windows in the block where the prime minister, Jens Stoltenberg, and his administration work were shattered.

The police official who spoke on condition of anonymity said the Oslo bombing occurred at 3.26pm local time, and the camp shootings began one to two hours later. The official said the gunman used automatic weapons and handguns, and that there was at least one unexploded device at the youth camp that a police bomb disposal team and military experts were disarming.

Seven people were killed by the blast in Oslo, four of whom have been identified. Nine or 10 people were seriously injured.